Analysis of gas diffusivity in coal using micro-computed tomography

Abstract

As coalbed methane (CBM) becomes an increasingly important resource in the global natural gas market, gas transport in CBM reservoirs remains a crucial research topic that has not been fully understood. Gas migration in coal matrix is identified as a diffusion process which is quantified by effective diffusivity. This paper aims to explore the diffusion process in coal and measure the effective diffusivity using X-ray microcomputed tomography (micro-CT) imaging. By visualising the krypton gas migration process using micro-CT imaging, we are able to simulate the gas transport and better understand the diffusion mechanism of adsorptive gas in coal. The effective diffusivity of krypton that we obtain in the coal matrix is estimated to be 3.4 × 10−11 m2/s which falls in the mid-range of reported value in the literature. In this paper, we develop a method based on using micro-CT imaging and high X-ray attenuated gas to visualise gas transport and quantitatively analyse diffusion process in porous media. The experiment can be conducted at in situ pressure conditions to mimic CBM storage and production process. This method is ought to provide more insights into the gas diffusion mechanism in coal while preserving the original rock structure.